A visible light polymerizing resin becomes widely used in the dental clinical field. As a photopolymerization initiator, since U. K. Patent No. 1,408,265 (Patent Document 1), a camphorquinone having a maximum absorption wavelength of 468 nm is mainly used. A hydrogen abstraction type polymerization initiation mechanism is known in which camphorquinone absorbs light to form a photoexcitation complex (exciplex) with an amine compound which is a hydrogen donor, generating an amine-derived free radical.
However, since camphorquinone has an extremely great b value expressing a tinge of yellow in the CIE Lab color specification system because of its absorption wavelength and absorbance, it is problematic to use for esthetic restorative in the dental field.
Furthermore, since an absorption wavelength of a polymerization initiation system with a combination of camphorquinone and aromatic tertiary amine is within a visible region, hardening is initiated by ambient light such as a fluorescent lamp and a dental lamp in a dental clinic room, exhibiting a high hardening speed and a short usable time. Therefore, there is a problem that a viscosity of a liquid of a bonding agent or a composite resin paste is increased upon treatment or therapy, making therapeutic operation difficult.
Patent-publication No. 2740829 (Patent Document 2) reports that 2,3-dioxobicyclo[2.2.1]heptane derivative (camphorquinone derivative) having a maximum absorption wavelength in 400 to 650 nm is a photopolymerization initiator that exhibits excellent photopolymerization ability upon polymerization of a compound having an ethylenic double bond.
Acylphosphine oxides disclosed in U.S. Pat. No. 4,265,723 (Patent Document 3) and U.S. Pat. No. 4,298,738 (Patent Document 4) is an α-cleaving type photoinitiator and generates a [—(O═)C.] radical and a [. P(═O)<] radical by C—P cleavage of an acylphosphine oxide group [—(C═O)—(P═O)<] in a molecule due to photo-irradiation. As acylphosphine oxide, so-called “bisacylphosphine oxides” that is a compound having two acyl groups in a molecule has also been put into practice, and it is disclosed in U.S. Pat. No. 4,792,632 (Patent Document 5), U.S. Pat. No. 5,721,292 (Patent Document 6) and U.S. Pat. No. 5,965,776 (Patent Document 7).
These acylphosphine oxides and bisacylphosphine oxides are widely used in the photopolymerization industry because of their great photopolymerization activity in a ultraviolet or near-ultraviolet region. Recently, they are used also in the dental field.
However, acylphosphine oxides exhibit excellent photo-hardening property with a halogen lamp (Hal) irradiator, but there is a disadvantage that they never become hardened have with a dental irradiator in a visible region of 430 to 500 nm, especially with a light-emitting diode (LED) irradiator and a xenon lamp (Xe) irradiator.
Japanese Patent No. 3442776 (Patent Document 8) proposes a visible light photopolymerizable adhesive consisting of a camphorquinone derivative, an acylphosphine oxide compound, at least one kind aliphatic amine and a radical polymerizable monomer.
However, a photoinitiator obtained by physically mixing them does not solve the problem of a high b value which indicates a tinge of yellow by color tone from camphorquinone at all. And also, a hardenable composition replacing aliphatic amine with aromatic amine in the photoinitiator has a problem, for use in the dental clinical field, that a color tone of a hardened product exhibits bister and a usable time is remarkably short.
That is, a photopolymerization initiator which exhibits photopolymerization activity over a wide region from near-ultraviolet to visible region, manifests excellent photopolymerizability with a dental irradiator such as halogen, LED, and xenon, is excellent in color tone property of a hardened product, enhances physical properties, and can overcome an antinomy issue that “relaxed operation under high photo-hardening speed and ambient light”, and which can be widely used in the dental field and the photopolymerization industry is desired.
Patent Document 1: U. K. Patent No. 1,408,265
Patent Document 2: Patent Publication No. 2740829
Patent Document 3: U.S. Pat. No. 4,265,723
Patent Document 4: U.S. Pat. No. 4,298,738
Patent Document 5: U.S. Pat. No. 4,792,632
Patent Document 6: U.S. Pat. No. 5,721,292
Patent Document 7: U.S. Pat. No. 5,965,776
Patent Document 8: Patent Publication No. 3442776